1. Field of the Invention
The present invention relates to a ceramic color composition and a process for producing a curved surface glass sheet employing it.
2. Discussion of Background
A ceramic color paste for an automobile has been widely used, which is designed so that a ceramic color composition in the form of a paste is screen-printed on the periphery or the center portion of a window glass of an automobile and dried, and then baked during a step of heating the glass sheet for bending.
This ceramic color paste is used for such a purpose that when baked on the periphery of the glass sheet, it forms a colored opaque layer to prevent deterioration of a urethane sealant by ultraviolet rays or to prevent seeing through of e.g. heating wire terminals from outside the vehicle. As a composition for such a purpose, a composition is known wherein a various heat resistant colored pigment is mixed to a base material of a low melting point amorphous glass or crystallizable glass containing lead as the main component. As such a glass, the one comprising 53.24 wt % of PbO, 30.78 wt % of SiO.sub.2, 0.68 wt % Of ZrO.sub.2, 3.13 wt % of Al.sub.2 O.sub.3, 9.66 wt % of B.sub.2 O.sub.3 and 2.56 wt % of CdO is, for example, known (Japanese Unexamined Patent Publication No. 160845/1989). However, such a glass contains lead and cadmium and accordingly has a possibility of a problem from the aspect of pollution or recycling. On the other hand, the following composition is known as a glass containing no lead or cadmium (Japanese Unexamined Patent Publication No. 34537/1990).
Bi.sub.2 O.sub.3 : 25 to 45 wt % PA1 SiO.sub.2 : 25 to 35 wt % PA1 B.sub.2 O.sub.3 : 10 to 25 wt % PA1 Alkali metal oxides: 4 to 19 wt % PA1 ZrO.sub.2 : 0 to 3 wt % PA1 TiO.sub.2 : 0 to 5 wt % PA1 ZrO.sub.2 +TiO.sub.2 : 0.3 to 8 wt % PA1 Bi.sub.2 O.sub.3 : 45 to 65 wt % PA1 SiO.sub.2 : 25 to 36 wt % PA1 B.sub.2 O.sub.3 : 4 to 6 wt % PA1 TiO.sub.2 : 3 to 6 wt % PA1 Na.sub.2 O: 1 to 3 wt % PA1 K.sub.2 O: 0.5 to 2 wt % PA1 Li.sub.2 O: 2 to 6 wt %
Further, the following composition is also known (Japanese Unexamined Patent Publication No. 180730/1990).
Furthermore, U.S. Pat. No. 4,959,090 discloses a composition having silicon incorporated to a lead-alumina-borosilicate frit.
On the other hand, in recent years, a production system so-called an in-furnace bending method is employed as a bending method for a window glass for an automobile, for the purpose of improving the productivity and the bending precision, wherein a pressing apparatus or a bending and suction molding apparatus is provided in a heating furnace so that the bending operation is conducted in the furnace.
However, if the above-mentioned ceramic color paste is used for the in-furnace bending method, there will be a problem such that the ceramic color composition adheres to a heat resistant cloth such as a glass cloth which is commonly used on the surface of the pressing mold or a suction mold for bending, whereby so-called mold release tends to be poor, and the productivity will be low. On the other hand, the ceramic color paste containing no lead or cadmium usually has a large expansion coefficient, and when such a paste is baked on a glass sheet, the strength of the glass sheet tends to be impaired, and it has an additional problem such that the chemical durability of the ceramic color composition is inadequate.